Image enhancement is often undertaken to improve the quality, or emphasize particular aspects of, an acquired digital image. Image sharpening is one enhancement technique that emphasizes the edges of the subject within the image by high pass filtering the image. An edge in an image occurs where there is an abrupt change in intensity, or a discontinuity, between two regions within an image. Known image sharpening processes generally involve (1) detecting image edges in an acquired image array of gray-scale intensity pixel values and (2) sharpening the image by modifying pixels values of each detected edge.
Edge detection processing is a known technique and can be performed by calculating intensity gradients across the image. Gradient vectors calculated for first-order gradient operators, for example, yield the gradient magnitude and angle of the local intensity gradient at each pixel. Calculating gradient vectors in this manner involves convolving the acquired image array with two weighted windows, one giving the x Cartesian component of the gradient and the other giving the y Cartesian component (see Awcock G. W. and Thomas R., “Applied Image Processing”, McGraw Hill, Chapter 4, 1996). The calculated gradient magnitudes are low in value in uniform intensity regions of the image, and high in value in regions containing edges (i.e. where there is a non-uniform intensity region). The occurrence of a high gradient magnitude value, indicating a sudden intensity transition, is evidence of an edge discontinuity at that pixel, the edge being at a normal to the calculated gradient angle.
Image sharpening involves increasing the image contrast in the local vicinity of image edges by modifying pixel values of edge pixels. When an edge separates a darker region from a lighter region, for example, and higher pixel values are indicative of lighter pixel intensities, image sharpening may be undertaken by decreasing the pixel values of pixels that are adjacent to, and on the darker side of the edge. Similarly, pixel values would be simultaneously increased for pixels that are adjacent to, but on the lighter side of the edge. The resulting increase in contrast in the local vicinity of edges is often referred to as overshoot.
Unsharp masking is a well known image sharpening technique and is often incorporated into photo manipulation software such as the well known Adobe Photoshop (Trade Mark) or other equivalents, for enabling users to sharpen acquired images. This sharpening technique involves adding the second derivative (i.e. Laplacian) of an original image array, to that original image array, which produces a sharpened image array. Manual adjustment of the degree of sharpening can be performed by multiplying a “sharpening factor” by the second derivative, and subsequently adding the result to the original image pixel values. In this way, a higher sharpening factor results in a greater degree of sharpening and therefore higher contrast around the edges of the sharpened image.
Image sharpening can be undertaken automatically to known levels by an image acquisition means such as a photocopier, image scanner, digital camera or other like image acquisition means containing the appropriate software, or alternatively, it can be undertaken manually on a digital processing apparatus such as a personal computer (PC) using appropriate software after the image has been acquired.
Problems can occur when images are sharpened too much because excessive contrast can occur at the image edges thereby resulting in image “clipping” and/or “ringing” as a consequence of excessive overshoot of pixel level values at the edges. These over-sharpening effects are often readily discernible by the human eye. The actual amount of sharpening that can ideally be performed depends on a number of factors related to: the image acquisition including lighting conditions, the contrast in the original scene, the optical system through which the image was acquired, and the conditions under which the image is to be viewed including the resolution and the gamma of the monitor used to produce the image display. The eye of the viewer is usually the best judge of the correct amount of sharpening.